This invention relates generally to color flat panel displays and, more particularly, to an electroluminescent flat panel display with aging correction.
Electroluminescent displays are flat panel displays that emit light from pixel locations based on the level of signal applied to each pixel location. Organic electroluminescent displays employ organic thin films deposited at pixel locations to emit light. The intensity of the emitted light is proportional to current passing through the organic thin films. The color of the emitted light and the efficiency of the energy conversion from current to light are determined by the composition of the organic thin films.
FIG. 1 shows a cross section view of a typical prior art active matrix bottom emitting electroluminescent display such as an organic light emitting diode (OLED) flat panel display 10 of the type shown in U.S. Pat. No. 5,937,272, issued Aug. 10, 1999 to Tang. The OLED display 10 includes a transparent substrate 12 that provides mechanical support for the display device, a transistor switching matrix layer 14, a light emission layer 18 containing materials forming organic light emitting diodes, and a cable 20 for connecting circuitry within the flat panel display to video interface circuit 24, located on printed circuit board 26. The transparent substrate 12 is typically glass, but other materials, such as plastic, may be used. The transistor switching matrix layer 14 contains a two-dimensional matrix of thin film transistors (TFTs) 16 that are used to select which pixel in the OLED display receives image data at a given time. The thin film transistors 16 are manufactured using conventional semiconductor manufacturing processes, and therefore extra thin film transistors 16 may be used to form circuitry for a variety of uses. As taught in U.S. Ser. No. 09/774,221 filed Jan. 30, 2001, by Feldman et al., the presence of TFTs within an active matrix flat panel display allows functions other than display to be implemented on the same substrate as the display function, producing a system-on-panel. The OLED display is responsive to digital control signals and analog video signals generated by video interface circuit 24.
It is known to those skilled in the art that organic light emitting materials undergo an aging process, where changes in the materials cause the light output of a given material for a constant input current stimulus to change with age. This causes a given image signal to produce a different image as the materials age. However, users of organic electroluminescent displays expect a given image signal to produce the same image, regardless of the ages of the organic light emitting materials. Alternatively, users may expect a given image signal to produce a pleasing image, although not necessarily the same image, regardless of the ages of the organic light emitting materials. For example, a dimmer image, but with proper color balance, may be acceptable, rather than the same, brighter image.
P. Salam, in his paper xe2x80x9cOLED and LED Displays with Autonomous Pixel Matching,xe2x80x9d published in the SID 2001 Digest, pages 67-69, describes a closed-loop luminance control system that utilizes light sensors placed around the periphery of an OLED display for sensing pixel light output to feed back luminance information to a color correction circuit. A xe2x80x9cmonitoring modexe2x80x9d is used for the pixel light display when the display is not in use wherein a single or area of pixels, is addressed one color channel at a time, and the emitted light is detected to generate a control signal.
The signal from the light sensor undergoes analog-to-digital conversion, and a processor calculates the measured light value and stores it. During normal image display, the display controller utilizes this stored luminance information to correct for non-uniformities in the light outputs from the pixels, which may occur due to aging. This method is complex because it requires a xe2x80x9cpixel luminance mapxe2x80x9d that must store information regarding each pixel, or each area of pixels, which may require a lot of memory. This can be expensive to implement, particularly for portable devices, which are often price sensitive. Additionally, the xe2x80x9cpixel luminance mapxe2x80x9d memories must be updated periodically, and therefore must be either a volatile or a rewritable volatile memory. Volatile memories typically consume more power than non-volatile memories, in order to maintain their contents. Non-volatile memories such as FLASH consume less power when not being written, but have a limited number of update cycles prior to failure. Therefore, a xe2x80x9cpixel luminance mapxe2x80x9d may be costly, power inefficient, and have limited life.
U.S. Pat. No. 6,081,073, issued Jun. 27, 2000 to Salam, describes a circuit and method for minimizing luminance and color variation in a light emitting diode matrix display, where light output is measured and stored, and a microprocessor or controller controls the measurement and correction process. Again, this method of performing luminance and color correction utilizes a memory map storing information about each display pixel, and therefore may be unnecessarily complex.
International application WO99/41732, published Aug. 19, 1999, by Matthies et al. describes several methods for correcting brightness due to OLED materials aging and pixel non-uniformity. These methods include measuring a physical aspect regarding light emitting pixels, performing calculations, and changing the current supplied to these light emitting pixels based on these measurements, in relation to stored accumulated current-values. This method directly modulates OLED current, and must therefore operate at the pixel level. However, display systems typically supply image information to displays using analog voltages and electronics within the display, or within the drivers that directly supply pixel current to pixels, to convert the voltage information into current. It is often desirable to modify these analog voltages, and not the currents to which the input analog voltages are converted.
There is a need therefore for an improved means of modifying analog video signals in a display device for the purpose of compensating for the aging of the corresponding organic light emitting materials that the signals drive, utilizing a simplified circuit.
The need is met according to the present invention by providing a display, that includes an electroluminescent display panel containing a light emitting material; a video interface circuit for producing an analog video signal for driving the display; an age circuit for supplying a signal representing the age of the light emitting material; an aging correction circuit responsive to the age signal for forming an analog aging correction signal, the aging correction circuit including, controller means responsive to the age signal for producing a digital correction value, and a digital to analog converter for converting the digital correction value to an analog correction signal; and a summing amplifier for summing the analog aging correction signal with the video signal.
The display according to the present invention is advantageous in that it exhibits a near constant luminance and/or color balance for given analog video voltages as the light emitting materials of an electroluminescent display age. The circuit is optimized for compensating analog video channels, and therefore is simpler, more cost efficient, and benefits from higher manufacturing yields than previous methods.